The present invention generally relates to methods and devices for detecting the presence or absence of high frequency signals in a system, and more specifically relates to a signal amplitude comparator.
The invention also relates to the use of a second threshold to make a circuit more sensitive to the peak amplitude of a signal applied to the circuit, and also relates to performing comparisons using devices operating large signal power law mode, thereby providing a better signal level than operation in linear mode for a given power level at a given offset.
There are numerous methods to detect the presence or absence of high frequency signals in a system. When the signal is primarily of alternating current, at frequencies where it is somewhat difficult to generate gain in a given semiconductor process, at a signal swing that is small compared to the Vt of the device and power is limited, the task becomes more difficult.
Dual differencing amplifiers have been applied to signal processing and comparison tasks with good success. However, at high frequencies, it is difficult to get good performance.
Square law operation of MOS transistors can generate good signal levels at high frequencies for signal detection. The presence of the direct current term as a result of product detection has an average value that can be constant over a wide range of frequencies. However, typical process variation makes it difficult to get a repeatable threshold value.
Signal amplitude comparator circuits can fall into two broad classes: average amplitude or RMS detectors and peak or near peak detectors. One aspect of the present invention uses a comparison circuit based on that which is disclosed in U.S. patent application Ser. No. 10/156,963, filed May 29, 2002, which is hereby incorporated herein by reference in its entirety, and the use of a second fixed threshold to make the circuit more sensitive to the peak amplitude of the signal applied. An additional facet of the circuit is that the comparisons are performed with devices operating large signal power law mode. This provides a better signal level than operation in linear mode for a given power level at a given offset. For Field Effect Transistors (FET""s), the operation in the comparison circuit is largely square-law.
A general object of an embodiment of the present invention is to provide a signal amplitude comparator which is configured to generate an output that accurately determines presence of a signal with a repeatable amount of amplitude.
Another object of an embodiment of the present invention is to provide a differential input circuit that is configured to generate an output current that is a non-linear function of an input voltage, and is configured to produce even order harmonic distortion over an input voltage range of interest.
Still another object of an embodiment of the present invention is to provide a signal amplitude comparator which uses a second fixed threshold to make the circuit more sensitive to the peak amplitude of the signal applied.
Yet another object of an embodiment of the present invention is to provide a signal amplitude comparator which provides that comparisons are performed with devices operating large signal power law mode.
Briefly, and in accordance with at least one of the forgoing objects, an embodiment of the present invention provides a signal amplitude comparator which includes a first differential input circuit that is biased, is configured to receive an input signal and is configured to generate a first output signal that is a non-linear function of the input signal, a second differential input circuit which is biased similarly to the first differential input circuit, is configured to receive a reference input signal and is configured to generate a second output signal that generally tracks process, temperature and supply variation, an amplifier which is connected to the first and second differential input circuits, a filter which is connected to the amplifier and a comparator which is connected to the filter. The amplifier is configured to amplify a signal difference between the first and second output signals and is configured to output a train of pulses to the filter if a peak of the input signal exceeds the reference input signal. A second reference signal is applied to the comparator, and the comparator is configured to receive and compare the first and second output signals, and is configured to generate an output which indicates whether the input signal exceeds a pre-determined threshold value. The signal amplitude comparator also includes a pair of linear (over the detection range of interest) input amplifiers which are connected to the first and second differential input circuits and are configured to receive and translate the input and reference input signals to levels suitable for the first and second differential input circuits.